# Beneficial fungal root endophyte Piriformospora indica inhibits bitter gourd mosaic complex disease incited by combined infection of tomato leaf curl, papaya ringspot, and cucumber mosaic viruses without compromising crop growth and yield by orchestrating ROS production and scavenging through retrograde signaling

## TL;DR

A beneficial fungus called Piriformospora indica reduces a complex plant disease in bitter gourd without harming crop growth or yield by managing harmful molecules in plant cells.

## Contribution

This is the first study showing that P. indica can simultaneously inhibit three viruses causing bitter gourd mosaic complex through retrograde signaling and ROS regulation.

## Key findings

- P. indica reduced BGMC incidence from 100% to less than 20% and disease severity from 84.17% to 9.02%.
- The fungus inhibited all three viruses by repressing their coat protein genes and virus titers.
- P. indica enhanced plant growth and fruit yield while modulating ROS and antioxidant gene expression.

## Abstract

Bitter gourd mosaic complex (BGMC)—caused by the combined infection of tomato leaf curl virus (ToLCV), papaya ringspot virus (PRSV), and cucumber mosaic virus (CMV)—poses a major threat to bitter gourd (Momordica charantia var. charantia L.) cultivation, resulting in severe yield losses. In this study, Piriformospora indica, a beneficial fungal root endophyte, was evaluated for its potential in managing BGMC. BGMC produced a wide range of symptoms. The presence of ToLCV, PRSV, and CMV in BGMC was confirmed serologically and molecularly. Piriformospora indica colonization in bitter gourd plants significantly reduced BGMC incidence from 100% to less than 20%, and the disease severity from 84.17 to 9.02%, when the viruses were inoculated by wedge grafting at 15 days after the colonization. Further, P. indica drastically reduced the disease severity from >75% to <30% under field conditions. Surprisingly, the fungus could significantly inhibit all three viruses responsible for BGMC, as evidenced by the repression of their coat protein genes and the virus titers in DAS-ELISA and PCR/RT-PCR. P. indica-colonization also enhanced growth and fruit yield. Biochemical analyses demonstrated that P. indica-colonization significantly suppressed ROS and H2O2 accumulation in the viruses-infected plants, while markedly enhancing the activities of antioxidant enzymes such as catalase, superoxide dismutase, and peroxidase (> 5-fold). Moreover, the concomitant downregulation of ROS and H2O2 marker genes (up to 12-fold less) and upregulation of antioxidant genes (up to 10-fold) located in the nucleus (WRKY40, MYB51, CML37, AGP5, CSD1, APX1), chloroplast (LOX2, PTOX, FSD1, FSD2), and mitochondria (HSPRO1, DIC2, PRX, MSD1, AOX2) of leaves clearly indicate the role of P. indica in orchestrating the regulation of ROS production and its scavenging through systemic retrograde signaling molecule(s) produced by the root endophyte. This premier study highlights the inherent ability of P. indica to diminish the complex symptoms of BGMC by simultaneous inhibition of ToLCV, PRSV, and CMV. ROS and H2O2-producing and scavenging genes located in the nucleus, chloroplast, and mitochondria are modulated by the retrograde signaling molecule(s) produced during the multipartite interaction involving the endophyte, plant, and the viruses.